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Topic: Science people: explain heat to me (Read 8723 times)

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Is the air really the same even temp or just at the thermostat, with areas near windows being colder for example in winter? Is the air moving more in winter? Are you getting more sunshine? Are your floors warmer?

Is the air really the same even temp or just at the thermostat, with areas near windows being colder for example in winter? Is the air moving more in winter? Are you getting more sunshine? Are your floors warmer?

No; yes; possibly; yes; definitely. Most of which suggest things I could/should be doing to get ready for next winter. I hadn't really thought about the floor temperature at all.

Yes as described above your thermostat is measuring the temperature one spot in the house.. so its likely you have cold spots.

Humidity makes a big difference as well. The human body is really a "Wet bulb" thermometer.. If the air is dryer sweat will evaporate faster and you will feel cooler.. Manely the wet bulb temperature is lower.

Its what we engineers call Psychrometry (science of water vapour basically).. Of course "physchology" probably has more to do with it..:)

Your temperature control in your home is a bang bang controller. That means it's on or off. The temperature must drop below the setting on the temp control before the heating kicks in. Your heating system has some built in lag before the heat actually gets up to the temperature you've set in. In the winter your home cools at a faster rate because it's colder outside than in the summer so the temperature lag goes on for longer which makes the house colder.

The upshot of all of this is that your house will actually be colder at the same temp setting in the winter. Check out this example for further illustration:

You have the thermostat set to turn on the heat when it drops below 17 . . .

Summer:17 - everything's good16 - temp dropping slowly15.5 - heater has kicked on and is now warming the house16 - heating17 - all good

This cycle of dropping to 15.5 then quickly heating to 17 will happen over and over again.

Winter17 - everything's good16 - temp dropping faster14 - heater has kicked on and is now warming the house15 - heating16 - heating17 - all good

This cycle of dropping to 14 and then slowly heating back to 17 will happen over and over again.

So, on average the actual temperature in your house is actually lower in the winter than the summer because of the increased speed of heat loss due to the greater temperature gradient between the inside and outside. Now, we could fix this temperature control problem by using a tuned PID controller . . . which would allow you to activate the heating before the temperature drops below your set value to keep the interior feeling warmer . . . but it's more complicated and expensive to do (and can be easily screwed up) so we don't.

Your house is probably also drafty. Those cold 0C drafts rolling across your skin make you say, "Brrrr!". When the drafts are 17C you say, "Ahhh, that feels nice". (psychology + thermodynamics = fun!)

This is why replacing the basement windows in our house in Quebec made the entire basement feel warmer, even though we didn't touch the thermostat. Apparently it was draftier down there than I realized.

Plus, in winter your exterior walls and floors are probably colder than 17C. Those cold surfaces radiate cold into the rest of the room. At least they're colder as long as your thermostat isn't mounted on an exterior wall.

Stand in front of your window on a sunny day, feel the heat. The temperature might be a constant 17C, but I'm sure you feel the difference. This is basically Radiant Heat.

Next step is to think of it coming from your walls, you probably can't actively notice because it's a pretty small amount. If you can then your insulation really sucks. In the summer there is heat coming in and "shining" on you. In the winter there isn't. Whenever there's a temperature differential the heat is going somewhere. From the sun to the earth and from outside to inside your walls.

Another popular way of thinking about it those heat imaging cameras. Those cameras are measuring heat waves (or lack of waves) coming from objects. So if a camera 20 feet from the source is capable of distinguishing heat waves why couldn't your body feel it?

I excelled at Heat and Mass transfer in Engineering. It's not in your mind, its a real phenomenon.

Your temperature control in your home is a bang bang controller. That means it's on or off. The temperature must drop below the setting on the temp control before the heating kicks in. Your heating system has some built in lag before the heat actually gets up to the temperature you've set in. In the winter your home cools at a faster rate because it's colder outside than in the summer so the temperature lag goes on for longer which makes the house colder.

The upshot of all of this is that your house will actually be colder at the same temp setting in the winter. Check out this example for further illustration:

You have the thermostat set to turn on the heat when it drops below 17 . . .

Summer:17 - everything's good16 - temp dropping slowly15.5 - heater has kicked on and is now warming the house16 - heating17 - all good

This cycle of dropping to 15.5 then quickly heating to 17 will happen over and over again.

Winter17 - everything's good16 - temp dropping faster14 - heater has kicked on and is now warming the house15 - heating16 - heating17 - all good

This cycle of dropping to 14 and then slowly heating back to 17 will happen over and over again.

So, on average the actual temperature in your house is actually lower in the winter than the summer because of the increased speed of heat loss due to the greater temperature gradient between the inside and outside. Now, we could fix this temperature control problem by using a tuned PID controller . . . which would allow you to activate the heating before the temperature drops below your set value to keep the interior feeling warmer . . . but it's more complicated and expensive to do (and can be easily screwed up) so we don't.

Thermodynamics is fun!

I'd say in the summer it probably works more like this:Summer:17 - everything's good18 - temp going up because it is slightly hotter outside18.5 - AC kicks on18 - getting cooler17 - all good

So you're actually experiencing an average more around 18 in the spring or summer rather than 16 or 15 as in the winter months.

Furthermore your temperature may lag lower near the windows and depending on where your thermostat is located it can take a longer time for it to realize that the temperature has dropped. So you have both lag time between thermostat hitting the right temp and telling furnace to turn on and turning on, and between colder areas influencing your thermostat in the first place.

Plus if you are multiple stories with a single zone thermostat and air system then you may find that the top floor is generally colder/warmer in winter/summer because the bottom floor will be more insulated against the extremes than the top.

Here's where the physics comes into play a bit more, the sun doesn't heat the air, rather it heats the ground then it radiates as infrared back into the air causing a lagging effect with the Earth which is why the Winter Solstice occurs in December but January and February are colder. This is also the reason why the hottest time is in the afternoon rather than at midday. With this understanding we can also understand that South facing windows in the winter months can help WARM your house because of the solar energy coming in.

Next part is how equilibrium happens, it depends on the temperature difference between inside and outside temps, so your house will cool slower when the outside temp is closer to the inside temp. The same is true for warming the house, but a major difference is that the sun's energy can be coming in straight through your windows and therefore bypassing the ambient temperature equilibrium. Just like how it can feel 10* cooler in the shade because you aren't having the sun pounding down on you.

Basically what is most likely happening is 1) you have more sunlight during the day to get into your house during the Spring, thus warming your house longer, and because the temperatures are the same the equilibrium is working slower than the energy coming in causing your house to warm up, and 2) the lag in temperature to the other extreme, averaging 18 instead of 16, so really a 2*C difference (3.6*F) rather than actually comparing 17*C, and3) the humidity difference may be a factor

Yes as described above your thermostat is measuring the temperature one spot in the house.. so its likely you have cold spots.

Humidity makes a big difference as well. The human body is really a "Wet bulb" thermometer.. If the air is dryer sweat will evaporate faster and you will feel cooler.. Manely the wet bulb temperature is lower.

Its what we engineers call Psychrometry (science of water vapour basically).. Of course "physchology" probably has more to do with it..:)

Frank

Good point - I wonder if more advanced thermostats like the Nest take humidity into account and calculate a "real feel" temperature. Nest might do it unintentionally since it will "learn" that you like it warmer in the winter (I suspect, I honestly don't know much about it besides that it is fancy looking and expensive).

Where I live (St. John's, NL) it never gets warm enough for air conditioning. But everything else y'all are saying is relevant and helpful. I've already made some changes over the past year that have benefited from some of the things you describe (e..g., moving more winter activities to the south side of the house, adjusting my work schedule to spend more warm/bright times of day at home). Now I need to work on drafts and floors (because insulation is too big a project for the number of years I'll stay in this house).

It's funny, I feel the opposite way as OP. In the winter we keep the temp at 70, summer = 76. It sometimes feels warm in the winter when it's on 70 and I'll bump it down a few degrees. If we kept it set on 70 in the summer (when it's 90-100 and humid outside), I swear I would grow icicles. Walking from the 90's outside to 76 feels like stepping into a freezer (oh so nice!).

There are also the physiological changes that happen in your body as the seasons change. In winter, your body constricts all the blood vessels in your extremities so that you don't lose heat as fast, but it makes it harder to keep your extremities warm (though your core is less likely to be cold, just your fingers, toes and ears). In the summer, you body expands all the blood vessels in your extremities because its easier to lose body heat from sweat when you have a lot of blood by the surface of your skin. Thus, you have more blood in your fingers and you feel warmer even if the temperature is the same around you.

Also if your thermostat reads 17C in the winter the floor, walls, and surrounding air are likely colder than the measured reading (assuming you have your thermostat on an inner wall). Where as if it is 17 degree outside and 17 degrees inside your house there is likely no temperature gradient, so it actually is several degrees warmer (as experienced by your body) even when the thermostat reads the same. Plus all the other reasons already mentioned.

I can attest to that. I spent a lot of time outdoors this winter (walking ~1 hr to/from school twice per day) and the cold didn't really bother me. Now that it's summer, I'm still outside a decent amount, and once you're outside for 20-30 minutes, the heat doesn't seem as bad.

I've also figured out that getting into a hot car to drive a half of a mile then walking another few minutes is way worse than just walking the half mile and skipping the car. Add in strapping/unstrapping a toddler in a car seat each time you enter or exit the vehicle and it's a no brainer to walk the 0.5 miles.

There are also the physiological changes that happen in your body as the seasons change. In winter, your body constricts all the blood vessels in your extremities so that you don't lose heat as fast, but it makes it harder to keep your extremities warm (though your core is less likely to be cold, just your fingers, toes and ears). In the summer, you body expands all the blood vessels in your extremities because its easier to lose body heat from sweat when you have a lot of blood by the surface of your skin. Thus, you have more blood in your fingers and you feel warmer even if the temperature is the same around you.

There are also shorter-term physiological changes induced by exercise (and possibly other things). Your body produces waste heat just from living, and more heat when you use muscles - that's why exercise makes you hot & sweaty - and the heat-producing mechanisms continue to work at an elevated rate after you stop exercising. (The body is replacing energy stores in the muscles, rebuilding & strengthening muscle &c in response to the stress of exercise, etc, all of which creates heat.) So differences in heat perception - whether it's too hot, too cold, or a Goldilocks' 'just right' - can depend on what you've been doing.

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper. Well actually, anything below 29 C I feel a bit chilly but being out in the sunshine definately helps :-)

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

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In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

I don't think the sun is irrelevant. On a dark cold day/night when the thermostat reads 17C your walls and windows will be a couple degrees colder than 17, and on a sunny day when your thermostat reads 17C your walls and windows may be a couple degrees warmer and radiating heat. Even without you directly absorbing the sunlight you may be experiencing several degrees of temperature difference at the same thermostat reading.

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

I don't think the sun is irrelevant. On a dark cold day/night when the thermostat reads 17C your walls and windows will be a couple degrees colder than 17, and on a sunny day when your thermostat reads 17C your walls and windows may be a couple degrees warmer and radiating heat. Even without you directly absorbing the sunlight you may be experiencing several degrees of temperature difference at the same thermostat reading.

Why wouldn't the thermostat experience that same radiating heat and adjust?

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In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

I don't think the sun is irrelevant. On a dark cold day/night when the thermostat reads 17C your walls and windows will be a couple degrees colder than 17, and on a sunny day when your thermostat reads 17C your walls and windows may be a couple degrees warmer and radiating heat. Even without you directly absorbing the sunlight you may be experiencing several degrees of temperature difference at the same thermostat reading.

Why wouldn't the thermostat experience that same radiating heat and adjust?

The thermostat is only an on/off controller. It clicks on once temperature drops below a particular threshold. If a wall has significant thermal mass like brick or concrete it can prolong (extra cold absorbing heat at night) or shorten (extra warmth radiating heat absorbed by the sun) the undershoot period before the furnace clicks on and begins to restore temperature.

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

I don't think the sun is irrelevant. On a dark cold day/night when the thermostat reads 17C your walls and windows will be a couple degrees colder than 17, and on a sunny day when your thermostat reads 17C your walls and windows may be a couple degrees warmer and radiating heat. Even without you directly absorbing the sunlight you may be experiencing several degrees of temperature difference at the same thermostat reading.

Why wouldn't the thermostat experience that same radiating heat and adjust?

The thermostat is only an on/off controller. It clicks on once temperature drops below a particular threshold. If a wall has significant thermal mass like brick or concrete it can prolong (extra cold absorbing heat at night) or shorten (extra warmth radiating heat absorbed by the sun) the undershoot period before the furnace clicks on and begins to restore temperature.

There are other reasons too, depending on the design of the thermostat. For example, older ones are essentially a metal coil. Metal doesn't absorb infrared as well as the human body. That's just one example.

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

I don't think the sun is irrelevant. On a dark cold day/night when the thermostat reads 17C your walls and windows will be a couple degrees colder than 17, and on a sunny day when your thermostat reads 17C your walls and windows may be a couple degrees warmer and radiating heat. Even without you directly absorbing the sunlight you may be experiencing several degrees of temperature difference at the same thermostat reading.

Why wouldn't the thermostat experience that same radiating heat and adjust?

It does adjust. While its adjusting on a hot day it absorbs heat, on a cold day it releases heat. When your outside walls are warmer than the inside heat will pass through and pass on to you. On a cold day heat goes outwards. So when you and the thermostat are experiencing the radiant heat you "feel" like you're warming up.

The difference between heat and temperature is subtle, everyone knows them but its hard to say what the difference is. The OP originally phrased the question in terms of temperature, which neglected how heat feels. What you feel is heat transfer, which is the amount of energy transferring between two objects. Temperature doesn't describe how heat moves between objects.

A neat experiment for the difference is to gather a few rods of different materials. For example use glass and copper rods, 12" long (steel, wood and others are all fun). Put one end on the stove, hold the other ends, and crank the heat. You'll notice the copper rod will get hot, heat transfer. The glass won't, it's a poor conductor of heat. The stove ends might be at 200C, in your hand the glass is 37C, that's temperature. The Copper burns your hand, that's heat. That's a 4th grade experiment on Thermodynamics- heat in motion.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

It doesn't feel the same because what you perceive is heat transfer, not temperature.

In the winter (outside temperature 0 Celsius), if my house is 17 C (63 F) I need to wear a sweater and I feel cold. Now that it's spring (outside temperature 17 C), my house is 17 C and I'm lolling about in shorts and a t-shirt.

Why?

(I acknowledge that part of the answer might need to come from the psychologists rather than the engineers...)

Could it also be the fact that spring = sunny and winter = no sun? I know that on a sunny 25 C day I feel nice and warm but on a cloudy rainy 25 C day I need to put on a jumper.

I think that's OP's point - if it's the exact same temperature, shouldn't you feel the same? The sun is irrelevant, other than it's making it get to that temperature, but if you're comparing two times with the same temperature, why doesn't it feel the same?

I don't think the sun is irrelevant. On a dark cold day/night when the thermostat reads 17C your walls and windows will be a couple degrees colder than 17, and on a sunny day when your thermostat reads 17C your walls and windows may be a couple degrees warmer and radiating heat. Even without you directly absorbing the sunlight you may be experiencing several degrees of temperature difference at the same thermostat reading.

Why wouldn't the thermostat experience that same radiating heat and adjust?

Because it is stationary on the inner wall and not moving around through the temperature gradient like people do. It's also not smart enough to understand there is a temperature gradient and compensate, it just turns on/off at a specific temp.

The thermostat is only an on/off controller. It clicks on once temperature drops below a particular threshold. If a wall has significant thermal mass like brick or concrete it can prolong (extra cold absorbing heat at night) or shorten (extra warmth radiating heat absorbed by the sun) the undershoot period before the furnace clicks on and begins to restore temperature.

There are other reasons too, depending on the design of the thermostat. For example, older ones are essentially a metal coil. Metal doesn't absorb infrared as well as the human body. That's just one example.

Because it is stationary on the inner wall and not moving around through the temperature gradient like people do. It's also not smart enough to understand there is a temperature gradient and compensate, it just turns on/off at a specific temp.

I think you guys are getting hung up on the word thermostat. Replace with thermometer, and same question. It's weird that a thermometer in the sun in the winter afternoon can show 60 degrees and it feels warm whereas a thermometer in the sun in the summer morning can show 60 degrees and it feels cold.

It being an on/off controller or metal coil or stationary on a wall and not moving around or temperature being a gradient are all irrelevant to this phenomenon.

It doesn't feel the same because what you perceive is heat transfer, not temperature.

Ah, finally an answer that might be correct. Thanks Prairie Practicality and Galaxy.

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The thermostat is only an on/off controller. It clicks on once temperature drops below a particular threshold. If a wall has significant thermal mass like brick or concrete it can prolong (extra cold absorbing heat at night) or shorten (extra warmth radiating heat absorbed by the sun) the undershoot period before the furnace clicks on and begins to restore temperature.

There are other reasons too, depending on the design of the thermostat. For example, older ones are essentially a metal coil. Metal doesn't absorb infrared as well as the human body. That's just one example.

Because it is stationary on the inner wall and not moving around through the temperature gradient like people do. It's also not smart enough to understand there is a temperature gradient and compensate, it just turns on/off at a specific temp.

I think you guys are getting hung up on the word thermostat. Replace with thermometer, and same question. It's weird that a thermometer in the sun in the winter afternoon can show 60 degrees and it feels warm whereas a thermometer in the sun in the summer morning can show 60 degrees and it feels cold.

It being an on/off controller or metal coil or stationary on a wall and not moving around or temperature being a gradient are all irrelevant to this phenomenon.

I don't think anyone is getting hung up. If I place a thermometer in on my couch by the window it will read colder in the winter than in the summer (at the same reading on my HVAC thermostat) because of the temperature gradient. Heat is coming from the outer walls in summer, and heat is going the opposite direction in the winter. This makes for very different real thermometer readings at various points in my house. Sometimes as much as 10*F in one direction (ie my thermostat is set at 64, but one of my spare bedrooms is actually at 54). It's not as dramatic as that in most of my living area, but the effect is the opposite in the summer. The area by the vents is the coolest, until the entire living room around the thermostat gets cool. Then the house starts warming up from the outside in until the thermostat reading is high enough to trigger the AC again.

I guess the crux of my post is that if I set my thermostat at 70*F all year, I would be experience different temps at my breakfast table (67* in the winter, and 73* in the summer).